An image forming apparatus includes a plurality of formation units which are aligned along a sheet transport belt, and sequentially forms each color image on a sheet transported on the belt by each formation unit. In such an image forming apparatus, a technique called registration is employed to reduce or prevent deviation (positional deviation) of an image forming position of each color on the sheet between the formation units, or a technique called density correction to reduce or prevent density change of a toner image of each formation unit.
The image forming apparatus which employs these technique includes an optical sensor having a light emitting section and a light receiving section. The light emitting section emits light to the belt, and the light receiving section receives the reflected light and outputs a light receiving signal corresponding to the amount of received light. Moreover, when performing the registration or density correction, a mark is formed on the belt by each formation unit. Then, the position or the density of the mark is determined by reading the variation of reflectance (amount of reflected light) between the belt surface and the mark surface based on the light receiving signal from the light receiving section, and deviation of the image forming position or density is corrected based on the determination result.
However, the determination precision of the position or density of the mark may vary according to the use condition and the like of the image forming apparatus. Nevertheless, since the same mark is always used in the technique described above, there is a possibility that the efficiency in time for determining the mark position or the efficiency in the amount of toner used or the like might deteriorate.